How to check that it is real iphone

What makes you concerned it is not an authentic iPhone?

You can enter your serial number here --> https://checkcoverage.apple.com/

Easiest way I have found -

1) visit https://checkcoverage.apple.com

2) Put your serial number and check the if the serial Number matches up ( you can find the serial number in settings > general > about

another way is to hook it up with a cable to iTunes.